This invention relates to an improved method of semiconductor device manufacture and more particularly to a method of treating the surfaces of glass sealed feed-through terminals to provide for improved bonding in the subsequent assembly of a semiconductor device.
Semiconductor devices employed in high current or high voltage applications, commonly referred to as power devices, generate a substantial amount of heat in the operation of the device. In order to rapidly dissipate this heat and thereby allow the semiconductor unit to operate at rated power without damage, it is customary to employ a heat conductive member of substantial mass as a heat sink to transfer heat from the semiconductor unit during operation of the device. This heat sink may also function as a mounting base or support for the other elements of the device; for example, the die, cap, terminals, etc. Thus a common power device configuration utilizes a one piece mounting base of substantial thickness which provides a high rate of heat dissipation and easy mounting on electrical equipment. The mounting base may be provided with a raised portion or pedestal which serves as a holder for a semiconductor die or chip and at the same time serves with the remainder of the one piece base as one of the electrodes or terminals of the semiconductor device. Since the other terminals of the semiconductor device must be electrically insulated from this base, they are commonly inserted or "fed through" holes in this base using a glass collar which provides for electrical insulation and simultaneously forms a hermetic seal.
In completing the assembly of the semiconductor device, a critical and expensive operation is the forming of an electrical connection from the semiconductor diode chip to the glass sealed feed-through terminal. This electrical connection is generally formed by bonding a wire from the surface of the semiconductor die to the feed through terminal. Various bonding methods including thermal compression bonding and ultrasonic bonding are employed to form the connection which must be mechanically strong and which must have high electrical conductivity. Because the semiconductor chip or die is relatively fragile, the bonding energy used to form the bonds to its surface must be carefully controlled to avoid damage to the semiconductor surface or the underlying junctions. In the past, one method which has been employed is to first form an ultrasonic bond to the semiconductor die and then in a second operation to form a connection to the feed through terminal using tweezer welding. The method achieves good mechanical and electrical connections since each separate attachment operation is adapted to its particular attachment point but it is costly because it requires two separate operations performed on two different bonding machines. In studying possible improved methods of manufacturing, the cost advantages of a single step bonding operation have been recognized but the implementation of this type operation has proved difficult because the limited bonding energies required to avoid damage to the semiconductor unit are too low to form good mechanical and electrical bonds to the feed-through terminal.